The present invention relates to a device for regulating the length of thread absorbed by a flat or circular knitting machine.
The notion of absorbed length of thread (ALT) per mesh, which was introduced by the Centre de Recherche de la Bonneterie is now well known and used by knitwear manufacturers. It is an important parameter which is taken into account in the regulation of knitting machines. The constancy of the ALT in the course of the knitting enables articles to be obtained whose dimensions will themselves be constant. The ALT is a function essentially of the adjustment of the dropping cams and of the conditions of supply of the thread. Now, for a same thread, the supply conditions will themselves be a function of the coefficient of friction of the thread: for a given adjustment of the dropping cams and adjustment of the thread-tighteners, if the coefficient of friction of the thread varies in the course of the knitting, the ALT thereof will be modified. These modifications will result in irregularities as regard the dimensions of the knitted article and its possible rejection.
To overcome this drawback, the simplest solution has been to install positive supply devices for each thread in the machine. The speed of these positive supply devices is regulated so as to deliver to the machine the length of thread corresponding to the desired ALT. Thus, whatever the coefficient of friction of the thread, the machine receives the predetermined amount of thread uniformly. This solution is not however envisageable for all types of machines. In fact, the positive supply device having a continuous operation can only be suitable in the case where the thread is distributed permanently to the needles; it is mainly suitable for circular knitting machines. On the other hand, this solution is not adapted to machines where the thread is distributed discontinuously to the needles, particularly flat knitting machines.
Another solution for overcoming the irregularities of the ALT in the course of knitting is to modify the tension of the thread supplying the machine. In fact, the variations in coefficient of friction of the thread will also be manifested by variations in the tension exerted on the thread by the various members upstream of the machine. Devices are already known intended to correct the variations in tension of the thread. Swiss Pat. No. 12 160/74 of Sept. 6, 1974 describes a device comprising a double-cup tensioning means between which the thread passes, and electromagnetic means having an action on the cups and causing the pressure exerted by the cups on the thread to be varied, this action being itself controlled by the opening or closing of the switch, caused by the friction of the thread on a guide element forming part of the switch.
In French Pat. No. 1 544 469 of Nov. 14, 1967, the variation in tension of the thread results in the movement of a roller and the action of the latter on a diaphragm pneumatic device controlling the approach or separation of the two cups of the tensioner. In French Pat. No. 71 40 701, the variation of the tension of the thread results in the movement of a rod fast to a cam which itself acts on a spring connected to the cups of the tensioning means.
Although they respond theoretically to the problem set the above-described devices have not really been applied to flat machines, either on account of resonance phenomena when it is mechanical and interdependant means which are employed, or on account of their lack of reliability, or on account of their lack of sensitivity or on account of their all or nothing action.
In addition, if the ALT is very certainly dependant on the tension of the thread supplying the machine and hence the variation of the ALT is an inverse function of the variation of this tension, it is important to note that the tension is a parameter which can vary almost momentarily: a very localised irregularity of oiling on the thread can result in a sudden variation in the tension. However, the variation in ALT which will be manifested by a fault in the knitted article is not that arising from these momentary jerks in the tension. What is useful for the quality of the finished product and what must hence be regulated, is the variation in the ALT due to a gradual change in the coefficient of friction of the thread or to a sudden variation in the average friction. The coefficient of friction can in fact vary of course from one material to another and, for the same material, from one numbering or from one presentation to another and, for the same type of thread, from one batch, from one spool, or from one color to another.
There has been found, and this is what forms the subject of the invention, a device which responds to the exigencies of knitwear manufacturers for regulating the ALT of circular and flat knitting machines, and which, consequently, enables also correction of the non-instantaneous variation in tension on a thread. This device comprises a tensioning means of known operation, whether this is a double cup tensioner, a bar tensioner, whether it is a braked rotary tensioner, or whether it is a tensioner acting by gripping the thread. It also comprises a first means for varying the tension exerted by the tensioner on the thread, a feeler element over which the thread passes and which is moved when the tension of the thread varies and a second means for controlling the action of the first means as a function of the movement of the feeler element, characterised in that the first means comprises a drive member whose rotation results in the movement of the tension generating member of the tensioner, and in that the second means comprises two switches, one controlling the placing in rotation of the drive member in one direction and the other controlling the placing in rotation in the other direction. The direction of rotation of the drive member is determined so that the tension generating member of the tensioner is moved in the direction of an increase in the tension of the thread in the case where it is the switch corresponding to a value of the ALT higher than the average normal value which has been actuated and conversely.
On the other hand, the rotation of the drive member is interrupted when, under the effect of the increase or of the decrease in the tension exerted on the thread by the movement of the tension generating member of the tensioner, and hence taking into account the consequent variation of the ALT, the feeler element is moved until it is no longer beyond or opposite the switch and comes back into the zone situated between the two switches. It is hence possible to decompose the space that the feeler element can scan into three zones. In the central zone bounded by the two contact switches, the movement of the feeler element does not result in any effect on the drive member; this zone corresponds to an acceptable variation in the ALT and the tension of the thread. On each side of this central zone are situated two zones where the presence of the feeler element results in the rotation of the drive member, for one of the zones in one direction, for the other zone in the other direction.
It is understood that with the device of the invention it will be easy, by moving one with respect to the other, each of these three zones, to obtain an accurate adjustment of the range of variation of the ALT around the average normal value which is acceptable, in the same way as the adjustment of the average normal value, as a function of the type of material, of thread, of batch and of spool.
Advantageously, the drive member whose rotation results in the movement of the tension-generating member of the tensioner comprises a motor with two directions of rotation. It may however be constituted by a motor only rotating in one direction, coupled to a reversing system, for example a rack, enabling the direction of rotation transmitted to be reversed.
The tensioner according to the invention is anyone of known tensioners. It may be taken particularly from among barrage tensioners comprising one, two or several elements in contact with the thread and where the tension exerted on the thread is a function of the contact arc between the thread and the elements which compose the bars. Among tensioners acting by gripping the thread where the tension exerted on the thread is a function of the pressure exerted by the movable gripping member on the thread; among braked rotary tensioners where the tension exerted on the thread is a function of the force exerted by the braking member on the rotating element driven by the thread. As tensioner acting by gripping the thread, may be mentioned in particular the two cup tensioner between which passes the thread and where the tension exerted on the thread is a function of the pressure exerted by a pressure member such as a spring on the two cups.
Switches controlling the rotation of the drive member in one or other direction are conventional electrical switches or preferably magnetic type ILS switches (flexible blade switches).
In the device according to the invention, the variation in the ALT results in the movement of the feeler element over which the thread passes, this movement being able to trigger the closing or opening of the switch. The feeler element comprises a thread-guide, of known type, and a rigid rod situated so that, on the movement of the feeler element, said rod comes into contact with electrical switches or opposite magnetic switches. In the case of magnetic switches of the ILS type, the rod will be matched to a magnetic mass.
The movement of the feeler element due to the variation of the ALT is a result of a variation in the length of travel of the thread between three points of which the two extremes are fixed, and the third situated between the two first is movable. It is this third point which is materialised by the thread-guide of the feeler element and which is moved as a function of the variations in the ALT and the tension exerted on the thread. If the ALT increases and hence the tension decreases, the length of the path tends to increase; if the ALT decreases and hence the tension increases, the length of the course tends to diminish. The third point which is moved to follow the variation in the length of the travel of the thread may be moved from above downwards or from below upwards for the same length variation.
These two possibilities have given two embodiments. In the first embodiment, the feeler element comprises a mass whose constant weight communicates by means of the thread-guide to the thread a certain constant tension, it is on this mass that the rigid rod which actuate the two switches is fixed. In the second embodiment, the thread-guide of the feeler element is fixed to the end of a lever oscillating around a fixed axle, the other end of said lever acting as a rigid rod and actuating the two switches. Advantageously, the end of the lever serving as a rigid rod is equipped with a counterweight movable along said lever, so as to regulate by simple movement of said counterweight the tension exerted on the thread by means of the thread-guide. Advantageously, the supports of the switches are fast to the axle around which the lever pivots, so as to enable the movement of said switches with respect to the rigid rod and hence the adjustment of the three zones by simple rotation of said supports around said axle.
Advantageously, a detection system is placed in the path of the thread downstream of the device according to the invention and upstream of the knitting machine, the detection system having the purpose of detecting if the thread is moved or not and blocking the operation of the device for regulating the ALT in the case where the thread would not be moved.